Bone is known to be a dynamic organ which is continuously remodelled by repeated formation and resorption so as to change its own morphology and maintain blood calcium levels. Healthy bone maintains an equilibrium between bone formation by osteoblasts and bone resorption by osteoclasts, and the bone mass is maintained constant. In contrast, when the equilibrium between bone formation and bone resorption is lost, abnormal bone metabolism such as osteoporosis occurs (Endocrinological Review, (1992) 13, pp. 66-80, Principles of Bone Biology, Academic Press, New York, (1996) pp. 87-102).
As factors which regulate bone metabolism, many systemic hormones and local cytokines have been reported, and these factors collaborate with one another to form and maintain bone (Endocrinological Review, (1992) 13, pp. 66-80, Endocrinological Review, (1996) 17, pp. 308-332). As a change in bone tissue due to aging, the occurrence of osteoporosis is widely known, but the mechanism of its occurrence encompasses various factors such as a decrease in secretion of sex hormones and an abnormality in the receptors for the hormones, variation in cytokine expression locally in bone, expression of aging genes, and osteoclast or osteoblast differentiation failure or dysfunction, and thus, it is difficult to consider it as a simple age-related physiological phenomenon. Primary osteoporosis is largely divided into postmenopausal osteoporosis due to a decrease in secretion of estrogen and senile osteoporosis due to aging, but advancement of basic research on the mechanisms of regulation of bone formation and bone resorption is essential to elucidate the mechanism of its occurrence and to develop a therapeutic agent therefor.
Osteoclasts are multinucleated cells derived from hematopoietic stem cells, and by releasing chloride ions and hydrogen ions on a bone surface to which osteoclasts adhere, osteoclasts acidify a gap between the bone surface and the osteoclasts and also secrete cathepsin K which is an acid protease or the like (American Journal of Physiology, (1991) 260, C1315-C1324). This causes degradation of calcium phosphate, activation of acid proteases and degradation of bone matrix proteins, resulting in bone resorption.
Osteoclast precursor cells have been found to be differentiated into osteoclasts by stimulation with RANKL (receptor activator of NF-κB ligand) expressed on the cell membrane of osteoblasts/stromal cells present on the surface of bone (Proceedings of the National Academy of Science of the United States of America, (1998) 95, pp. 3597-3602, Cell, (1998) 93, pp. 165-176). It has been revealed that RANKL is a membrane protein produced by osteoblasts/stromal cells, its expression being regulated by a bone resorption factor, RANKL induces differentiation of osteoclast precursor cells into multinucleated osteoclasts, and the like (Proceedings of the National Academy of Science of the United States of America, (1998) 95, pp. 3597-3602, Journal of Bone and Mineral Research, (1998) 23, S222). Further, knockout mice devoid of RANKL have been found to develop an osteopetrosis-like disease, and therefore, RANKL has been proved to be a physiological osteoclast differentiation-inducing factor (Nature, (1999) 397, pp. 315-323).
As drugs for treating bone metabolism diseases or shortening the duration of treatment, bisphosphonates, active vitamin D3, calcitonin and derivatives thereof, hormone preparations such as estradiol, SERMs (selective estrogen receptor modulators), ipriflavone, vitamin K2 (menatetrenone), PTH (parathyroid hormone) preparations, calcium preparations and the like are used. However, these drugs do not always exhibit a satisfactory therapeutic effect and the development of an agent with a more potent therapeutic effect has been demanded.
The cell membranes of immune cells are covered with a dense coating of various glycans, such as sialic acid, which are recognized by various glycan-binding proteins. Sialic-acid-binding immunoglobulin-like lectins (hereinafter referred to as “siglecs”) are a family of type I membrane proteins which recognize sialylated glycans and bind thereto. Many siglecs are expressed on the cell membranes of immune cells and recognize sialic acid similarly present on the cell membranes of immune cells and regulate cell interaction or cell function and are considered to be involved in the immune response (Nature Reviews Immunology, (2007) 7, pp. 255-266). However, there are also a lot of siglec molecules whose physiological functions have not been elucidated yet. Siglec-15 (Sialic-acid binding immunoglobulin-like lectin 15) is a molecule which has been newly reported to belong to the Siglecs (Glycobiology, (2007) 17, pp. 838-846) and is identical to a molecule called CD33L3 (CD33 molecule-like 3). This molecule is highly evolutionarily conserved from fish to humans and has been found to be strongly expressed in dendritic cells and/or macrophages of human spleen and lymph nodes. Further, as a result of a binding test using a sialic acid probe, it has also been found that human Siglec-15 binds to Neu5Acα2-6GalNAc, and that mouse Siglec-15 binds to Neu5Acα2-3Galβ1-4Glc in addition to Neu5Acα2-6GalNAc (Glycobiology, (2007) 17, pp. 838-846). Until recently, the physiological role of Siglec-15 was not revealed, however, it has been reported that the expression of Siglec-15 increases with the differentiation and maturation of osteoclasts, and the differentiation of osteoclasts is inhibited by decreasing the expression of Siglec-15 by RNA interference (WO 2007/093042). However, the effect of an anti-Siglec-15 antibody on osteoclast differentiation has not been elucidated yet.